The invention relates to the field of magnetic nuclear resonance, and in particular to an improved apparatus for nuclear spin tomography wherein a high-frequency transmitter is decoupled from a high-frequency receiver using a cableless local coil device.
Devices for generating images of an object to be investigated using magnetic nuclear resonance are well-known. Such devices are commonly used to generate images of a human body or part thereof to assist in diagnosis of various physical ailments. In a typical prior art device, a body to be imaged is placed in a powerful homogenous magnetic field, referred to as the "basic field," which aligns the nuclear spin of atomic nuclei in the body. Such fields are particularly effective on hydrogen atom nuclei, or protons, bonded to water molecules. The nuclei are excited into a precessional movement by high-frequency exciting pulses. At the end of such an exciting pulse, the atomic nuclei precess with a frequency that depends on the strength of the basic field. Then, after a predetermined relaxation period, the spin of the nuclei cause them to oscillate back in the preferred direction determined by the basic field.
Using a computer and/or other measurement technology to analyze the integral high-frequency nuclear signals, an image of a layer of the body can be generated from the three-dimensional spin density or the distribution of the relaxation times. Linear field gradients may be used to allocate the nuclear resonance signal detectable as a consequence of the precessional movement at the point that it occurs. The quality of the image may be enhanced by superimposing suitable gradients on the basic field and controlling them so that excitation of the nuclei is confined to a single layer of the body to be imaged. Imaging based on these physical effects is known generally as nuclear spin (NS) or nuclear magnetic resonance (NMR) tomography.
A high-frequency device for nuclear spin tomography is described in German patent DE-OS 35 00 456. A high-frequency transmitter is used to excite nuclear spin in a body to be investigated. A high-frequency receiver is then used to detect high-frequency B-field components produced by the nuclear spin excitation. The receiver includes a local coil device which is electrically-insulated from the other components of the receiver for placement on the body to be imaged. The receiver also includes an external receiving antenna surrounding the local coil device and coupled inductively therewith.
In known high-frequency devices of the type to which the present invention is directed, two different high-frequency components are required to create an image; namely, a high-frequency transmitter and a high-frequency receiver. A high-frequency receiver with a local coil device is often used for such applications, especially where body areas of relatively limited extent are to be imaged. Such a local coil device typically has a surface or local coil which is placed on the part of the body to be imaged, such as a vertebra, the middle ear, or an eye. In the simplest implementation, the local coil consists of a circular antenna loop made of wire. A capacitor is used to bridge a break in at least one point, and the device is switched by high-frequency signals. Such a local coil device may be used to pick up the high-frequency signals generated by the nuclear spin excitation as a corresponding B-field.
Devices employing high-frequency excitation of nuclear spin require a transmitter with at least one antenna to produce the excitation. A suitable high-frequency transmitter antenna may be designed as a "whole-body resonator," as described in Wilhelm Durr et al., High Frequency System for Nuclear Spin Tomography, ntz Archiv, Vol. 11, No. 5, at 237-243 (1989) and in European publication EP-B-0 073 375. Such an antenna is formed as a round hollow-conductor antenna which surrounds the body to be imaged.
In known high-frequency nuclear spin tomography systems, the high-frequency signals resulting from the nuclear spin excitation are typically conducted through a feed or connecting cable running from the investigation area out to signal-processing equipment. In such devices, the feed cable itself can degrade system performance by inhibiting transmission of the high-frequency signals. Moreover, disturbances in the form of sheath waves may form on the feed cable, which can result in an undesirably high power density in the area of the body being imaged. One solution to these drawbacks of cable-based devices has been to electrically insulate the local coil device from the other components of the receiver.
In the high-frequency receiver described in German patent DE-OS-35 00 456, described above, a certain component of the B-field emitted by the body being imaged is picked up by a local coil device having at least one antenna loop, thereby generating a current in the local coil device. The associated B-field is then detected by a corresponding receiving antenna surrounding the body being imaged, and thus surrounding the local coil device as well. A resulting signal induced in the outer antenna is fed by known methods to electronics coupled to the outer antenna for further processing.
A principle drawback of such inductively-coupled devices lies in the fact that the measurement signal is weakened by the inductive coupling between the local coil device and the outer receiving antenna. The noise component of the signal produced by the outer receiving antenna as the signal is fed to the electronic signal processing equipment is thus undesirably high. This problem, manifested as a low signal-to-noise (S/N) ratio, is especially critical when either the field-sensitive components of the local coil device are relatively small or the outer receiving antenna exhibits a relatively poor Q-factor.